Asymmetric Write Scheme for Magnetic Bit Cell Elements

ABSTRACT

Asymmetric switching is defined for magnetic bit cell elements. A magnetic bit cell for memory and other devices includes a transistor coupled to an MTJ structure. A bit line is coupled at one terminal of the bit cell to the MTJ structure. At another terminal of the bit cell, a source line is coupled to the source/drain terminal of the transistor. The bit line is driven by a bit line driver that provides a first voltage. The source line is driven by a source line driver that provides a second voltage. The second voltage is larger than the first voltage. The switching characteristics of the bit cell and MTJ structure are improved and made more reliable by one or a combination of applying the higher second voltage to the source line and/or reducing the overall parasitic resistance in the magnetic bit cell element.

TECHNICAL FIELD

The present teachings relate, in general, to magnetic memory and, inparticular, to an asymmetric write scheme in magnetic bit cell elements.

BACKGROUND

The progress and development of magnetoresistive random access memory(MRAM) technology has increased the viability of selecting MRAM forvarious embedded & standalone nonvolatile memory applications. Insteadof storing data as an electric charge, MRAM stores data as magneticmoment. MRAM sensing exploits the magnetoresistive effect that occurs inmagnetic tunnel junctions (MTJs). FIG. 1A is a block diagramillustrating a magnetic tunnel junction (MTJ) 10. The MTJ 10 includes amagnetic layer 101, an insulator layer 103, and a magnetic layer 102, anupper contact 104 and a lower contact 105 coupled to a substrate 100.The magnetic layers 101-102 may be constructed from a variety oftransitional-metal ferromagnets and other magnetic materials, includingcobalt-iron, or the like, or also from combinational layers of varioussynthetic antiferromagnetic (SAF) and antiferromagnetic (AFM) layers.The insulator layer 103 may also be constructed from a variety ofinsulating materials, such as magnesium oxide or the like. The currentor voltage level applied to the MTJ 10 will control the relativemagnetic orientations of the magnetic layers 101-102. In one instance,applying a particular current or voltage level will cause the magneticorientation in the magnetic layer 101 to be anti-parallel to themagnetic orientation of the magnetic layer 102. Similarly, anothercurrent or voltage level will cause the magnetic orientations of themagnetic layers 101-102 to be the same or parallel.

When the magnetic orientations of the magnetic layers 101-102 areparallel, electrons will be more likely to tunnel through the insulatorlayer 103 than when the magnetic orientations are anti-parallel. Thismagnetoresistive effect causes the resistance of the MTJ 10 to be highwhen the magnetic orientations of the magnetic layers 101-102 areanti-parallel and low when the magnetic orientations are parallel. Bymeasuring this resistance, the value of the data stored by the MTJ 10can be determined.

In the configuration of many MTJ memories, such as the MTJ 10, one ofthe magnetic layers usually has a fixed magnetic orientation while theother layer is a free floating layer which is capable of having itsmagnetic orientation change according to the application of theparticular current or voltage.

FIG. 1B is a block diagram illustrating a programmable spin-logic device11 based on a single MTJ element 106. A spin-logic device, such asprogrammable spin-logic device 11, is a configuration of one or moremagnetoresistive devices into various logic elements, such as logicgates. The logic functionality is often obtained by manipulating theswitching thresholds of the magnetoresistive devices and places suchdevices in a particular configuration. The illustrated programmablespin-logic device 11 is merely one example of such a spin-logic devicethat may be configured as various logic gates, such as AND, OR, NAND,NOR, and the like.

At the core of the programmable spin-logic device 11 is the MTJ element106. The MTJ element 106 is made up of magnetic layers 107 and 108 withan insulation layer 109 placed between the two magnetic layers 107 and108. The operation of the MTJ element 106 as a programmable element issimilar to the operation described with respect to the MTJ 10 (FIG. 1).The relative magnetic orientations of the magnetic layers 107 and 108determine the data stored in the MTJ element 106. Writing the data tothe MTJ element 106 involves application of sufficient current orvoltage to switch the magnetic orientation of the free magnetic layer.In order to create a programmable logic element, three input contacts110-112 are provided coupled to the magnetic layer 107 with an outputcontact 113 coupled to the magnetic layer 108.

In practice, the input contacts 110-112 are operated with positive ornegative currents, ±I_(A), ±I_(B), and ±I_(C), of equal magnitude. Themagnetic layers 107 and 108 have a magnetism, ±M₁ and ±M₂, respectively,where the ± reflects the magnetic orientation of either of the magneticlayers 107 and 108. The magnetic layers 107 and 108 also have differentcoercive fields, H_(C1) and H_(C2), respectively, where H_(C2) isgreater than H_(C1). Individually, application of any of the currentsI_(A), I_(B), and I_(C) is insufficient to generate enough of a magneticfield to reverse either M₁ or M₂ However, when I_(A) and I_(B) areapplied together, enough of a magnetic field is generated to reverse M₁of the magnetic layer 107, while the coercive field, H_(C2), is stilllarge enough to resist reversal. When all three currents are appliedtogether, the combined magnetic field is sufficient to reverse both M₁and M₂. Therefore, by manipulating the initial set-up relationshipbetween the magnetic layers 107 and 108, AND and OR gates may beconfigured using the MTJ element 106 and only the input contacts 110 and111, and, if the third input contact 112, is used, NAND and NOR gatesmay be configured.

FIG. 2 is a schematic diagram illustrating the circuit equivalent of aspin torque transfer (STT) MTJ device 20. The STT MTJ device 20 may beimplemented as a memory, such as an MRAM, or as some other type ofspin-logic device, such as an AND gate. STT technology uses spin-alignedor polarized electrons to directly torque the physical system.Specifically, as electrons flow into a pinned thick magnetic layer, theybecome polarized. When these polarized electrons come near to the freelayer, they will exert a torque tending to change the magneticorientation of the nearby layer.

Because of its inherent resistance, the MTJ 200 is represented by aresistor in the schematic diagram. This resistance will cause a voltagedrop, V_(MTJ), over the MTJ 200. The MTJ 200 is coupled on one side to abit line 202 and on the other side to the drain contact of thetransistor 201. The transistor 201 is coupled at its source contact to asource line 203 and at its gate contact to a word line 204. In order towrite data to the STT MTJ device 20, a voltage, V_(WL), is applied tothe word line 204. V_(WL) is designed to be sufficient to turn on thetransistor 201 in operational conditions.

The value written to the MTJ 200 will depend on how the STT MTJ device20 loads the transistor 201. When there is a voltage, V_(BL), on the bitline 202 as the word line 204 is activated, and the source line 203 hasa relative low voltage, a logical ‘1’ will be written to the MTJ 200.The current direction in the STT MTJ device 20 with this biasingarrangement produces a current flow from the bit line 202 toward thesource line 203. This current direction through the MTJ 200 sets up theappropriate relative magnetic layer magnetic orientations that representa logical ‘1’. In contrast, when a voltage, V_(SL), is applied to thesource line 203 as the word line 204 is activated, and the bit line 202has a relatively low voltage, the current flow in the STT MTJ device 20is in the opposite direction (i.e., from the source line 203 toward thebit line 202). This current direction establishes the appropriatemagnetic layer magnetic orientations to reflect a logical ‘0’ in the MTJ200. Because the inherent resistance in the MTJ 200 causes a sourceloading effect in the write ‘0’ process, it is more difficult to write a‘0’ in this type of configuration. Moreover, power is wasted because thevoltages will be applied to the STT MTJ device 20 longer in order totrigger the state change in the MTJ 200 that produces the ‘0’.

It should be noted that the MTJ 200 may be coupled into the STT MTJdevice 20 in various different ways. As illustrated in FIG. 1A, one ofthe magnetic layers in an MTJ, such as MTJ 200, will often have a fixedmagnetic orientation, while the other magnetic layer has a free floatingmagnetic orientation. The current flow direction that will generallyyield the highest resistance in the MTJ 200 is when the current flowtravels from the fixed or reference magnetic orientation layer to thefree layer. Thus, in the configuration illustrated in FIG. 2, the freefloating magnetic layer side of the MTJ 200 is connected to thetransistor 201, while the fixed magnetic layer side of the MTJ 200 iscoupled to the bit line 202. Thus, in the write ‘0’ process when the bitline 202 is biased with a relatively low or zero voltage with respect tothe source line 203, the current flows in the direction from the sourceline 203 to the bit line 202. The higher resistance with this directionof current flowing from the free magnetic layer to the fixed orreference magnetic layer results in a higher voltage drop, V_(MTJ),across the MTJ 200, which increases the source loading affect on thetransistor 201, which makes it more difficult to actually write the ‘0’to the MTJ 200. In alternative configurations, where the free magneticlayer is coupled to the bit line 202 and the reference magnetic layer iscoupled to the transistor 201, the process for writing a ‘1’ would bemore difficult.

FIG. 3 is a schematic diagram illustrating a magnetic memory 30. Themagnetic memory 30 includes an array 300 of multiple MTJ memory units301. The multiple MTJ memory units 301 are arranged in columns 302within the array 300. The ellipsis 306 in the lines of the columns 302represent the existence of multiple additional MTJ memory units 301within the columns 302. Each of the multiple MTJ memory units 301includes an STT MTJ structure 309 (represented as a resistor) and atransistor 310. The multiple MTJ memory units 301 are coupled to sourcelines 307 and bit lines 308. The multiple MTJ memory units 301 are alsocoupled to word lines 305 that trigger a write operation when asufficient voltage is applied. In order to select the particular memorycell on which to write data, a series of column switches 304 are inplace for each of the columns 302. A single set of source and bit linedrivers 303 are used to drive each of the source lines 307 and bit lines308 of the array 300. When a write command is received, an address isreceived along with it, which, when decoded, allows the magnetic memory30 to open or close the appropriate ones of the column switches 304. Theclosed ones of the column switches 304 provide voltage from the sourceand bit line drivers 303 to the appropriate ones of the source lines 307and the bit lines 308 corresponding to the memory cells designated bythe decoded address. Thus, the voltage provided by the source and bitline drivers 303 will only be applied to the appropriate memory cellassociated with the address.

BRIEF SUMMARY

Various embodiments of the present teachings are directed to anasynchronous switching scheme for magnetic bit cell devices. Examplemagnetic bit cells include a transistor coupled to an STT MTJ structure.At one terminal of the bit cell, a bit line is coupled to the STT MTJstructure. At another terminal of the bit cell, a source line is coupledto the source/drain terminal of the transistor. The bit line is drivenby a bit line driver that provides a first voltage to the bit line. Thesource line is driven by a source line driver that provides a secondvoltage to the source line. The second voltage is larger than the firstvoltage. In an MRAM array configuration, the switching characteristicsof the bit cell and STT MTJ structure is improved and made more reliableby one or a combination of applying the higher second voltage to thesource line and/or reducing the overall bit line and source lineparasitic resistance.

Representative embodiments of the present teachings are directed tomagnetic bit cell write circuits. Such write circuits include a firstwrite driver applying a first voltage, a second write driver applying asecond voltage that is higher than the first voltage, and at least oneMTJ structure coupled at one terminal to the first write driver andcoupled at another terminal to the second write driver, wherein the MTJstructure receives the first voltage to change from a first state to asecond state and receives the second voltage to change from the secondstate to the first state.

Further representative embodiments of the present teachings are directedto MRAM devices that include a plurality of memory columns. Each ofthese memory columns has at least one magnetic bit cell. The MRAMdevices also have a plurality of source lines. Each of these sourcelines is associated with a corresponding column of the memory columnsand is coupled to one terminal of the magnetic bit cell of thecorresponding column. The MRAM devices also have a plurality of bitlines. Each of these bit lines is associated with the correspondingcolumn and coupled to another terminal of the magnetic bit cell of thecorresponding column. The MRAM devices also have a plurality of firstdrivers. Each of these first drivers is coupled to a correspondingsource line and applies a first driver voltage to change the magneticbit cell from a first state to a second state. The MRAM devices alsohave a plurality of second drivers. Each of these second drivers iscoupled to a corresponding bit line and applies a second driver voltageto change the magnetic bit cell from the second state to the firststate.

Still further representative embodiments of the present teachings aredirected to methods for writing to an MTJ structure of a magnetic bitcell element. These methods include receiving a write signal on a wordline associated with the MTJ structure, detecting write data to bewritten to the MTJ structure in response to the write signal, andreceiving a first voltage on a bit line coupled to one terminal of theMTJ structure in response to the write data being a first value. Thefirst voltage causes the MTJ structure to change from a first state to asecond state. The methods also include receiving a second voltage on asource line coupled to another terminal of the MTJ structure in responseto the write data being a second value. The second voltage is higherthan the first voltage and causes the MTJ structure to change from thesecond state to the first state.

Additional representative embodiments of the present teachings aredirected to methods for writing to an MTJ structure of a magnetic bitcell element. These methods include the steps of receiving a writesignal on a word line associated with the MTJ structure, detecting writedata to be written to the MTJ structure in response to the write signal,and receiving a first voltage on a bit line coupled to one terminal ofthe MTJ structure in response to the write data being a first value. Thefirst voltage causes the MTJ structure to change from a first state to asecond state. The methods also include receiving, in response to thewrite data being a second value, a second voltage on a source linecoupled to another terminal of the MTJ structure. This second voltage ishigher than the first voltage and causes the MTJ structure to changefrom the second state to the first state.

Further representative embodiments of the present teachings are directedto systems for writing to an MTJ structure of a magnetic bit cellelement. These systems include means for receiving a write signal on aword line associated with the MTJ structure, means, executable inresponse to the write signal, for detecting write data to be written tothe MTJ structure, means, executable in response to the write data beinga first value, for receiving a first voltage on a bit line coupled toone terminal of the MTJ structure, the first voltage causing the MTJstructure to change from a first state to a second state, and means,executable in response to the write data being a second value, forreceiving a second voltage on a source line coupled to another terminalof the MTJ structure, the second voltage higher than the first voltageand causing the MTJ structure to change from the second state to thefirst state.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription that follows may be better understood. Additional featuresand advantages can be described hereinafter, which form the subject ofthe claims of the disclosure. It should be appreciated by those skilledin the art that the conception and specific aspects disclosed may bereadily utilized as a basis for modifying or designing other structuresfor carrying out the same purposes of the present disclosure. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the technology of the disclosure as setforth in the appended claims. The novel features, which are believed tobe characteristic of the disclosure, both as to its organization andmethod of operation, together with further objects and advantages, canbe better understood from the following description when considered inconnection with the accompanying figures. It is to be expresslyunderstood, however, that each of the figures is provided for thepurpose of illustration and description only and is not intended as adefinition of the limits of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present teachings, reference isnow made to the following description taken in conjunction with theaccompanying drawings.

FIG. 1A is a block diagram illustrating a magnetic tunnel junction(MTJ).

FIG. 1B is a block diagram illustrating an MTJ spin-logic device.

FIG. 2 is a schematic diagram illustrating a circuit equivalent of aspin torque transfer (STT) MTJ device.

FIG. 3 is a schematic diagram illustrating a magnetic memory.

FIG. 4 is a schematic diagram illustrating an MTJ column circuitequivalent of a column in the magnetic memory of FIG. 3.

FIG. 5 is a hysteresis graph illustrating the current flow through anSTT MTJ memory cell as a function of the bit cell biasing voltage.

FIG. 6 is a hysteresis graph illustrating the voltage drop across theMTJ structure (V_(MTJ)) as a function of the bit cell bias voltage.

FIG. 7 is a representative schematic diagram of a memory cell configuredaccording to one embodiment of the present teachings.

FIG. 8 is a performance record for an STT MTJ MRAM cell configuredaccording to one embodiment of the present teachings.

FIG. 9 is a schematic diagram illustrating a magnetic memory configuredaccording to one embodiment of the present teachings.

FIG. 10 is a performance record for an STT MTJ MRAM cell configuredaccording to one embodiment of the present teachings.

FIG. 11 is a graph of switching characteristics for an MTJ memory designT₁.

FIG. 12 is a graph of switching characteristics of a first MTJ memorydesign and a second MTJ memory design configured according to oneembodiment of the present teachings.

FIG. 13 is a logic diagram illustrating a cell selection circuitconfigured according to one embodiment of the present teachings.

FIG. 14 is a logic diagram illustrating a cell selection circuitconfigured according to one embodiment of the present teachings.

FIG. 15 is a block diagram illustrating a magnetic bit cell deviceconfigured according to one embodiment of the present teachings.

DETAILED DESCRIPTION

Magnetic bit cell elements, such as those used in MRAM, spin-logicdevices, or the like may be used in systems that maintain multipleinternal networks for power savings purposes. These systems includedevices such as mobile devices, mobile phones, and the like. The corenetwork of such a device is generally considered the circuitry thatoperates the core functionality of the device. The device may also havean input/output (I/O) network, which handles all external communicationbetween the device and external components or devices. The core networkwill communicate with the I/O network in order to transmit or receivesignals external to the device. Often times, the I/O network willoperate at a different, higher voltage level than the core network. Thehigher voltage may be used to drive the components that consume morepower, such as transmitters, receivers, and the like. In such instances,the core network communicates with the I/O network through multiplelevel shifters which shift the voltage levels between the two networks.

In such devices, an MRAM or possibly a spin-logic device is often partof the core network. Thus, the voltage provided to these elements isbased on the lower core voltage. As noted above, the source loadingeffect in STT MTJ device often makes it more difficult to write a ‘0’ tothe memory or programmable part of the cell (when the fixed or referencemagnetic layer or magnetic layer having the higher coercive field iscoupled to the bit line). In operation, because these STT MTJ devicesare also generally powered with the lower core voltages, the write ‘0’difficulties can become even more acute.

FIG. 4 is a schematic diagram illustrating an MTJ column circuitequivalent 40 of a column 302 in the magnetic memory 30 of FIG. 3. Inorder to ensure proper operation of a memory cell, such as MTJ memoryunit 301 (FIG. 3), certain voltage drops will be maintained across theMTJ structure 309 (FIG. 3). The specific voltage drops will control theMTJ structure 309 (FIG. 3) switching between parallel and anti-parallelmagnetic orientations, thus, switching memory values. However, inoperation, parasitic resistances may cause insufficient voltages to beapplied at each terminal of the MTJ structure 309 (FIG. 3) and, evenbefore that, at the terminals of the transistor 310 (FIG. 3). Theseparasitic resistances are illustrated in the MTJ column circuitequivalent 40. In the entire length of the column 302 (FIG. 3), there isan equivalent parasitic resistance resulting from the source and bitline drivers 303 (FIG. 3)—a driver resistance 400, from column switches304 (FIG. 3)—a switch resistance 401, from the inherent resistance inthe conducting traces of the magnetic memory 30 (FIG. 3)—a conductingtrace resistance 402, from the transistor 310 (FIG. 3)—an XTORresistance 403, and then from the MTJ structure 309 (FIG. 3) itself—anMTJ resistance 404. Therefore, the voltages seen at each terminal of theMTJ structure 309 (FIG. 3) will be reduced by the voltage drops causedby each of the effective resistances. The resulting switching conditionson the MTJ structure 309 (FIG. 3) may, at various times, be inadequateto ensure proper operation, which affects the overall operation of theMTJ memory unit 301 (FIG. 3). Moreover, the voltage applied at theterminals of the transistor 310 (FIG. 3) may also not be sufficient toactivate the transistor 310 (FIG. 3). Because the proper operation isnot guaranteed with this configuration, operation of the magnetic memory30 (FIG. 3) will not be reliable.

FIG. 5 is a hysteresis graph 50 illustrating a current flow 500 throughan STT MRAM memory cell as a function of the bit cell biasing voltage501. The hysteresis graph 50 illustrated in FIG. 5 represents thecurrent flow 500 in the STT MRAM in which the MTJ free layer is coupledto the source line of the STT MRAM transistor, while the MTJ referencelayer is coupled to the bit line. The bit cell biasing voltage 501 isrepresented by the source line voltage (V_(SL)) minus the bit linevoltage (V_(BL)). At point 503, the current flowing through the MTJjumps from approximately 90 μA to approximately 130 μA at point 502.Thus, resistance has decreased in the MTJ indicating the MTJ switchingfrom the higher resistance state to the lower resistance state. Thisswitching point corresponds to a voltage of approximately 1.4 V at thesource line and 0 V at the bit line.

As the bit cell bias voltage 501 decreases, the current flowing throughthe MTJ eventually reverses direction. At point 505, the current flowingthrough the MTJ reaches approximately −130 μA. It then drops toapproximately −90 μA at point 504. Therefore, the MTJ switches from itslow resistance state to its higher resistance state at points 505/504.The bit cell bias voltage 501 at points 504/505 is approximately −700 mVon the bit line with 0 V on the source line.

In analyzing the switching characteristics of the MTJ, it can be seenfrom the hysteresis graph 50 that MTJ switching occurs at asymmetricvoltages. Thus, for MTJ switching to be completed, V_(BL) may be lowerthan 1 V and V_(SL) should be larger than 1.4 V. In many applications,it is less complex to provide symmetric biasing than asymmetric biasing.However, the limitations of MTJ structures would prevent suchconfigurations. Certainly, if V_(BL) and V_(SL) were both biased at avalue of 700 mV (−/+), the MTJ may switch from the lower resistancestate to the higher resistance state, but it will not switch from thehigh resistance state to the lower resistance state. Conversely, ifV_(BL) and V_(SL) were both biased at 1.4 V, the MTJ may switch from thehigh resistance state, but, at the other end of the spectrum, the MTJstructure may breakdown after switching states from low to highresistance. The point 506 represents the point at which the MTJstructure begins to breakdown. The voltage drop across the MTJ bit cellstructure at point 506 is approximately −1.4 V. These operatingconditions may get even worse, with breakdown occurring earlier orswitching occurring at different voltage drops with variations in theprocess corners. Therefore, in order to maintain reliable operation,symmetric biasing mechanism may not be used.

FIG. 6 is a hysteresis graph 60 illustrating a voltage drop across theMTJ structure (V_(MTJ) 600) as a function of the bit cell bias voltage601. The switching of the MTJ structure is shown to occur at points602/603 and at points 604/605. Again, as reflected in hysteresis graph60, the switch at points 602/603 occurs with V_(SL) at approximately 1.4V with V_(BL) at 0 V, and the switch at points 604/605 occurs withV_(BL) at approximately −700 mV with V_(SL) at 0 V. The V_(MTJ) 600 atthe points 602/603 switch is approximately −450 mV, at point 602, andapproximately −520 mV, at point 603. As the bit cell bias voltage 601increases beyond 1.5 V, the trend in the hysteresis graph 60 suggeststhat the V_(MTJ) 600 only slowly increases, with the slope of the curveappearing to approach zero before reaching a voltage drop of −600 mV.

Considering the switch at points 604/605, the V_(MTJ) 600 isapproximately 520 mV at point 604, and approximately 450 mV at point605. Beyond this switch at points 604/605, as a larger voltage isapplied at V_(BL), the corresponding value of the V_(MTJ) 600 continuesto increase at a steady rate. However, once the V_(MTJ) 600 reachesapproximately 1 V, at point 606, the danger of the MTJ structurebreaking down increases dramatically. When the MTJ structure breaksdown, it may no longer reliably be used as a memory circuit until thestructure exits the break down conditions. Therefore, in analyzing theswitching characteristics for an MTJ in the context of the V_(MTJ) 600,the circuit should attempt to limit V_(MTJ) 600 to an amount lower thanapproximately 1 V.

It should be noted that the voltage and current values disclosed withregard to FIGS. 5 and 6 and each of the other FIGURES provided for inthis application are merely examples and are not intended to limit thescope and application of the present teachings to any such values ormaterials which might reflect those values. The various embodiments ofthe present teachings may operate with any various types of materialthat reflect other values and still fall within the scope of thisdisclosure.

In order to address the switching issues experienced with MTJs, a newmemory configuration is presented that provides an asymmetric switchingscheme in which one of the bit/source lines is coupled to the corenetwork voltage, while the other source/bit line is coupled to the I/Onetwork voltage. In this configuration, the I/O voltage provides ahigher voltage than the core network voltage. FIG. 7 is a representativeschematic diagram of a memory cell 70 configured according to oneembodiment of the present teachings. The memory cell 70 illustrates thefurthest bit cell 700 in a column of a magnetic memory (not shown). Thebit cell 700 includes an MTJ structure 701 (represented as a resistor)and a transistor 702. A source line 703 is coupled to a source/drainterminal of the transistor 702, while a bit line 704 is coupled to aterminal of the MTJ structure 701.

The bit line 704 is driven by a bit line driver 705. The bit line driver705 operates within the core network providing core voltage levels tothe bit line 704. The source line 703 is driven by a source line driver706. The source line driver 706 operates to provide I/O network voltageto the source line 703. A core network buffer 707 and the source linedriver 706 communicate at an interface provided by a level shifter 709,which is capable of shifting voltage levels between the two differentvoltages of the core and I/O networks.

The memory cell 70 may reside in a magnetic memory such as the magneticmemory 30 (FIG. 3). In such an example implementation, the source andbit line drivers 303 (FIG. 3) would be modified to conform to theconfiguration of the bit line driver 705 and the source line driver 706.This would enable each of the source lines 307 (FIG. 3) with the higherI/O voltage. In the overall circuit of the memory cell 70, a parasiticsource resistance 711 and a parasitic bit resistance 712 still existbecause of the inherent resistance added by the source line driver 706,the bit line driver 705, column switches 709 and 710, the conductingtrace resistance 402 (FIG. 4), and the transistor 702. However, becausethe higher I/O voltage is applied to the source line 703, there will bea sufficient voltage level at the source/drain terminal of thetransistor 702 to turn it on and sufficient voltage to cause the MTJstructure 701 to switch states, even with the slightly increased voltagedrop across the parasitic source resistance 711 due to the higher I/Ovoltage.

FIG. 8 is a performance record 80 for an STT MRAM cell configuredaccording to one embodiment of the present teachings. The STT MRAM cellrelated to the performance record 80 is configured much like the memorycell 70 (FIG. 7), with the source line being coupled to a source linedriver enabled to provide a higher voltage than that provided on the bitline. The performance record 80 includes graphs of the current flowingthrough the MTJ structure, I(MTJ) 800, the bit line biasing voltage,V(BL) 801, the source line biasing voltage, V(SL) 802, and the word linevoltage, V(WL) 803, each as a function of the same testing time line.Beginning at point 804 and throughout the testing time line, the V(WL)803 is set to its high state. For purposes of the example illustrated inFIG. 8, the high state of the V(WL) 803 is 1.2 V. Thus, a write commandis present for the duration of the testing time line.

The V(SL) 802 begins at point 805 set to its high state. For purposes ofthe example illustrated in FIG. 8, the high state of the V(SL) 802 is1.8 V. This high state reflects a higher voltage level than the voltageavailable for the V(WL) 803. The V(BL) 801 begins at point 807 set toits low state. For purposes of the example illustrated in FIG. 8, thelow states for each of the V(WL) 803, the V(SL) 802, and the V(BL) 801is 0 V. Moreover, the high state of V(BL) 801 is 1.2 V. With the V(WL)803 activated in a write command, the source line biased at the V(SL)802 in its high state, and the bit line biased at the V(BL) 801 in itslow state, the I(MTJ) 800 is measured at the beginning point 809 to be100 μA. At point 810, the I(MTJ) 800 jumps to a current of 150 μA. Thissudden increase in current flow at I(MTJ) 800 is a result of theresistance in the STT MTJ structure decreasing, thus, indicating the MTJstructure has switched states.

At point 806, the V(SL) 802 switches to its low state, while the V(BL)801 switches to its high state at point 808. This change in bit cellbiasing causes the I(MTJ) 800 to reverse current direction, but stillremain at its high state at point 811. At point 812, the I(MTJ) 800jumps from the high current state to a low current state. This suddendecrease in current flow at I(MTJ) 800 is a result of the resistance inthe MTJ structure increasing, thus, indicating the MTJ structure hasagain switched states. This current signature continues in the I(MTJ)800 for the remainder of the testing time line. In applying a highervoltage level to the V(BL) 802, the MTJ structure may be switched morereliably. The time periods 813 and 814 represent the switching speed forswitching from the high resistance, at point 809, to the lowerresistance, at point 810, and for switching from the low resistance, atpoint 811, to the higher resistance, at point 812, respectively.

The parasitic resistances 711 and 712 (FIG. 7) have a voltage-reducingaffect, such that voltages applied at the terminals of a representativebit cell, such as the bit cell 700 (FIG. 7), will be less than the fullamount provided to the source and bit lines by the source and bit linedrivers. This reduction in voltage makes operation of the bit cell evenmore difficult. As illustrated above, a certain voltage differential isneeded to cause the MTJ structure, such as MTJ structure 701 (FIG. 7),to switch states. Additionally, before the MTJ structure switchesstates, the voltage relationships will need to be sufficient to activatethe bit cell transistor, such as transistor 702 (FIG. 7). Therefore,another way to increase the voltage applied at the bit cell terminals isto reduce the overall resistance between the source and bit line driversand the bit cell terminals.

FIG. 9 is a schematic diagram illustrating a magnetic memory 90configured according to one embodiment of the present teachings. Themagnetic memory 90 includes an array 900 of multiple memory bit cells901 configured in a series of columns 909. Each of the memory bit cells901 includes an MTJ structure 902 and a transistor 903, where the gateterminals of the transistors 903 are coupled to a word line 910. Thesource lines 905 and the bit lines 906 couple the memory bit cells 901to a driving location 904. In contrast to the magnetic memory 30 (FIG.3), which includes the column switches 304 and a single, shared sourceand bit line drivers 303 (FIG. 3), the magnetic memory 90 is configuredwith a high voltage driver 907 and a low voltage driver 908 for each oneof the source lines 905 and the bit lines 906, respectively. The highvoltage drivers 907 are coupled to the source lines 905, while the lowvoltage drivers 908 are coupled to the bit lines 906. The high voltagedrivers 907 and the low voltage drivers 908 also include selectionlogic, which provides the functionality previously supplied by thecolumn switches 304 (FIG. 3). By removing the column switches 304 (FIG.3) and the single, shared source and bit line drivers 303 (FIG. 3), thenet effect of the addition of the individual high voltage drivers 907and the low voltage drivers 908 is a significant reduction in resistancebetween the drivers and the memory bit cells 901. This reduction inresistance translates into a higher effective voltage applied at theterminals of the memory bit cells 901, which results in a more reliablewriting process for the memory bit cells 901.

It should be noted that the addition of each of the high voltage drivers907 and low voltage drivers 908 may increase the chip area used tointegrate a magnetic memory, such as the magnetic memory 90. However,the negative effects of the increased chip area are countered by theincreased performance benefits realized by reducing the total resistancein the memory. The column switches 304 (FIG. 3) provide significantresistance in the memory, not only caused by the resistance in any oneswitch, but, because the resistance of the column switches 304 (FIG. 3)is experienced in parallel, the total resistance in each of the columns302 (FIG. 3) is significantly higher than the resistance of a singleswitch. Furthermore, because of the location of the column switches 304(FIG. 3), the source and bit line drivers 303 (FIG. 3) needs to be morerobust, in order to account for the added resistance of the columnswitches 304 (FIG. 3). Without the column switches, each of theindividual high voltage drivers 907 and low voltage drivers 908 do notneed to be as robust as the source and bit line drivers 303 (FIG. 3),thus, adding less resistance and individually requiring substantiallyless chip area.

FIG. 10 is a performance record 1000 for an STT MRAM cell configuredaccording to one embodiment of the present teachings. The STT MRAM cellrelated to the performance record 1000 is configured much like themagnetic memory 90 (FIG. 9), with the source line being coupled to asource line driver enabled to provide a higher voltage than thatprovided on the bit line. The performance record 1000 includes graphs ofthe current flowing through the MTJ structure, I(MTJ) 1001, the bit linebiasing voltage, V(BL) 1002, the source line biasing voltage, V(SL)1003, and the word line voltage, V(WL) 1004, each as a function of thesame testing time line. Beginning at point 1005 and throughout thetesting time line, the V(WL) 1004 is set to its high state. For purposesof the example illustrated in FIG. 10, the high state of the V(WL) 1004is 1.2 V. Thus, a write command is present for the duration of thetesting time line.

The V(SL) 1003 begins at point 1006 set to its high state. For purposesof the example illustrated in FIG. 10, the high state of the V(SL) 1003is 1.8 V. This high state reflects a higher voltage level than thevoltage available for the V(WL) 1004. The V(BL) 1002 begins at point1008 set to its low state. For purposes of the example illustrated inFIG. 10, the low states of each of the V(WL) 1004, the V(SL) 1003, andthe V(BL) 1002 are 0 V. Moreover, the high state of V(BL) 1002 is 1.2 V,which is the same lower voltage available to the V(WL) 1004. With theV(WL) 1004 activated in a write command, the source line biased at theV(SL) 1003 in its high state, and the bit line biased at the V(BL) 1002in its low state, the I(MTJ) 1001 is measured at the beginning point1010 to be 100 μA. At point 1011, the I(MTJ) 1001 jumps to a current of150 μA. This sudden increased current flow at I(MTJ) 1001 is a result ofthe resistance in the MTJ structure decreasing, thus, indicating the MTJstructure has switched states.

At point 1007, the V(SL) 1003 switches to its low state, while the V(BL)1002 switches to its high state at point 1009. This change in bit cellbiasing causes the I(MTJ) 1001 to reverse current direction, but stillremain at its high current state at point 1012. At point 1013, theI(MTJ) 1001 jumps from the high current state to a low current state.This sudden decrease in current flow at I(MTJ) 1001 is a result of theresistance in the MTJ structure increasing, thus, indicating that theMTJ structure has again switched states. This current signaturecontinues in the I(MTJ) 1001 for the remainder of the testing time line.In applying a higher voltage level to the V(BL) 1002, the MTJ structuremay be switched more reliably. The time periods 1014 and 1015 representthe switching speed for switching from the high resistance, at point1010, to the lower resistance, at point 1011, and for switching from thelow resistance, at point 1012, to the higher resistance, at point 1013,respectively. The time periods 1014 and 1015 have been reduced incomparison with the switching speed time periods 813 and 814 (FIG. 8) asa result of the significant reduction in total parasitic resistancebetween the line drivers, such as the high voltage drivers 907 (FIG. 9)and the low voltage drivers 908 (FIG. 9), and the individual bit cells,such as bit cells 901 (FIG. 9)

In considering the switching characteristics of an STT MRAM design, theswitching of states follows a particular set of parameters. FIG. 11 is agraph 1100 of the switching characteristics for an STT MRAM design. Theswitching characteristics follow a characteristics curve, T₁,illustrated in the graph 1100. The characteristics curve T₁ addressesthe critical switching current, I_(C) 1101, as a function of theswitching time, t 1102. The memory is first designed to thecharacteristics at point 1103 of the characteristics curve T₁. Withthese functional characteristics, by applying a current, I_(C1), to thememory design, the magnetic memory will switch after a time, t₁. Inorder to increase the switching speed of the memory design to a time,t₂, a current of I_(C2) will need to be applied to the memory. Indesigning a memory system that would provide for this new switching timeof t₂ according to the characteristics curve T₁, the designers will needto address certain design trade-offs. For example, the existing devicesmay have power sources that are limited to supply only the current,I_(C1). Thus, to make the improvement in switching speed, the powersupplies will be replaced. This replacement may cost more, because ofthe higher power output requirements, or may take up more space, or willlikely use more power during operation. In a mobile device that operateson battery power, power consumption is a serious consideration.Therefore, the decrease in switching time may not be cost-effectiveconsidering the additional monetary and power costs the decrease mayrequire.

Instead of attempting to change the operation of a particular memorydesign, the various embodiments of the present teachings have changedthe design itself. With the change in the design, the overalloperational characteristics are changed. FIG. 12 is a graph 1200 of theswitching characteristics of a first memory design represented by thecharacteristics curve T₁ and a second memory design configured accordingto one embodiment of the present teachings. The characteristics curve T₁represents the same switching characteristics illustrated in FIG. 11. Atpoint 1103, the characteristics reflect a switching time of t₁ with theapplication of the current, I_(CF). The memory design configuredaccording to one embodiment of the present teachings has operationalcharacteristics reflected in the characteristics curve, T₂. The T₂memory design improves switching by increasing the voltage applied tothe associated source line and reduces the overall parasitic resistance,as described with respect to FIGS. 7 and 9. Because the memory designreflected by the characteristics curve T₂ has created a faster switchingmemory, the entire characteristic curve T₂ has shifted in time.Therefore, by providing the same critical current I_(CF), the new memorydesign switches at time t₂. By switching more quickly, the word line mayshut off more quickly, which saves power for the underlying system.

In designing the high voltage driver 907 (FIG. 9) of the magnetic memory90 (FIG. 9), switching logic is added in order to perform the switchingfunctionality previously provided by the column switches 304 (FIG. 3).FIG. 13 is a logic diagram illustrating a cell selection circuit 1300configured according to one embodiment of the present teachings. Thecell selection circuit 1300 provides switching functionality for highervoltage source line drivers configured according to one embodiment ofthe present teachings. Three signals are used in controlling theswitching functionality in the cell selection circuit 1300. A writesignal 1305 represents the signal received from the word line when amemory “write” is activated. The column-select (col-sel) signal 1306 isanother signal that comes from the word line as address information. Theaddress information is decoded to obtain the col-sel signal 1306. Inhandling the higher voltages, level shifters 1301 provide voltageconversions from the lower voltages of the magnetic memory system. Thewrite-data signal 1307 is a signal that represents the data that is tobe written to the memory cell.

Using these three signals, the write signal 1305, the col-sel signal1306, and the write-data signal 1307, the cell selection circuit 1300determines whether to bias its source line or not. The write signal 1305and the col-sel signal 1306 are input into a NAND gate 1302. Theresulting signal from the NAND gate 1302 is used with the write-datasignal 1307 as input to an OR gate 1303. The resulting signal from theOR gate 1303 is then processed through an inverting buffer 1304. Theinverting buffer 1304 will bias the source line with the higher voltagelevel when the resulting signal from the OR gate 1303 is a logical ‘0’,or will leave the source line at 0 V when the resulting signal from theOR gate 1303 is a logical ‘1’. Thus, when attempting to write a ‘0’ tothe memory cell, the source line will be biased at the higher voltagelevel, and when attempting to write a ‘1’ to the memory cell, the sourceline will be biased at 0 V. The entire operational characteristics ofthe cell selection circuit 1300 are provided below in Table 1. The ‘X’entries in Table 1 represent an instance where the result would notchange regardless of whether the signal was a ‘0’ or a ‘1’.

TABLE 1 Write Col-Sel Data SL 0 X X 0 X 0 X 0 1 1 0 1 1 1 1 0

FIG. 14 is a logic diagram illustrating a cell selection circuit 1400configured according to one embodiment of the present teachings. Thecell selection circuit 1400 provides switching functionality for lowervoltage bit line drivers configured according to one embodiment of thepresent teachings. The cell selection circuit 1400 uses the same threesignals used by the cell selection circuit 1300 (FIG. 13). The writesignal 1305 and the col-sel signals 1306 are used as input into a NANDgate 1403. The resulting signal from the NAND gate 1403 is used with thewrite-data signal 1307 as input to an OR gate 1405. The resulting signalfrom the OR gate 1405 is used as gate input to the p-type transistor1408 portion of a complementary metal oxide silicon (CMOS) configuredswitch 1407.

The write signal 1305 and col-sel signal 1306 are also used as input toa NAND gate 1404. The resulting signal from the NAND gate 1404 is usedalong with the write-data signal 1307 as input to an OR gate 1406. Theresulting signal from the OR gate 1406 is used as gate input to then-type transistor 1409 of the CMOS configured switch 1407. Based on howthe p-type transistor 1408 and the n-type transistor 1409 are biased,the cell selection circuit 1400 will either bias the bit line with thelower voltage level or leave the bit line at 0 V. Thus, when attemptingto write a ‘1’ to the memory cell, the bit line will be biased at thelower voltage level, and when attempting to write a ‘0’ to the memorycell, the bit line will be biased at 0 V. The entire operationalcharacteristics of the cell selection circuit 1300 are provided below inTable 2. The ‘X’ entries in Table 2 represent an instance where theresult would not change regardless of whether the signal was a ‘0’ or a‘1’.

TABLE 2 Write Col-Sel Data BL 0 X X 0 X 0 X 0 1 1 0 0 1 1 1 1

It should be noted that the cell selection circuits described andillustrated with respect to FIGS. 13 and 14 are merely examples of thecircuit configurations that may be incorporated into the higher voltagesource line drivers and the lower voltage bit line drivers. Variousadditional embodiments of the present teachings may utilize differentlogic configurations to implement memory bit cell selection.

FIG. 15 is a block diagram illustrating a magnetic bit cell device 1500configured according to one embodiment of the present teachings. Themagnetic bit cell device 1500 includes an internal device section 1501powered by an external power source 1502. The internal device section1501 includes an array of bit cell elements, such as the bit cellelements 1503-1-1503-N. Each of the bit cell elements 1503-1-1503-N hasa bit line (BL), a source line (SL), and a word line (WL), powered,respectively, through SL drivers 1504-1-1504-N, BL drivers1505-1-1505-N, and a WL driver 1509. In accordance with the teachingspresented herein, the embodiment depicted in FIG. 15 provides a highervoltage on the SL through the SL drivers 1504-1-1504-N. This highervoltage is provided by the external power source 1502 through a chargepump 1506. The charge pump 1506 is able to step up or step down thevoltage received from the external power source 1502 in order to supplythe appropriate higher voltage to the SLs of the bit cell elements1503-1-1503-N. By utilizing the charge pump 1506, the magnetic bit celldevice 1500 is able to use a single voltage supplied from the externalpower source 1502 to generate and supply the different voltages used forthe source lines.

In various alternative embodiments, either one or both of the chargepumps 1507 and 1508 may be implemented in the internal device section1501. For example, if the source lines are to be provided with a firstvoltage, and both the bit lines and word lines are to be provided with asecond voltage, where the first voltage is higher than the secondvoltage and the voltage supplied by the external power source 1502 isdifferent than both the first and second voltages, the charge pumps 1506and 1507 would take the voltage from the external power source 1502 andcreate the first voltage (by the charge pump 1506) and the secondvoltage (by the charge pump 1507). In a separate example, if the wordline was to be provided with a third voltage, then the third voltagewould be created by the charge pump 1508 using the voltage supplied bythe external power source 1502.

It should be noted that in selected alternative embodiments, where thevoltage to be applied to any of the source, bit, or word lines is equalto the voltage supplied by the external power source 1502, thecorresponding charge pump of the charge pumps 1506-1508 may not beincluded in the internal device section, in which case, the voltagewould be supplied to the corresponding drivers, such as the SL drivers1504-1-1504-N, the BL drivers 1505-1-1505-N, and/or the WL driver 1509,directly from the external power source 1502.

Although specific circuitry has been set forth, it will be appreciatedby those skilled in the art that not all of the disclosed circuitry isrequired to practice the invention. Moreover, certain well knowncircuits have not been described, to maintain focus on the invention.Similarly, although the description refers to logical “0” and logical“1” in certain locations, one skilled in the art appreciates that thelogical values can be switched, with the remainder of the circuitadjusted accordingly, without affecting operation of the presentinvention.

The improved bit cell elements can be included in mobile devices, suchas a portable computers, cell phones, hand-held personal communicationsystems (PCS) units, portable data units such as personal dataassistants, fixed location data units such as meter reading equipment,set top boxes, music players, video players, entertainment units,navigation devices, or computers.

Although the present teachings and their advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the technologyof the teachings as defined by the appended claims. Moreover, the scopeof the present application is not intended to be limited to theparticular aspects of the process, machine, manufacture, composition ofmatter, means, methods and steps described in the specification. As oneof ordinary skill in the art will readily appreciate from thedisclosure, processes, machines, manufacture, compositions of matter,means, methods, or steps, presently existing or later to be developedthat perform substantially the same function or achieve substantiallythe same result as the corresponding aspects described herein may beutilized according to the present teachings. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A magnetic bit cell write circuit comprising: a first write driverapplying a first voltage; a second write driver applying a secondvoltage that is higher than said first voltage; and at least onemagnetic tunnel junction (MTJ) structure coupled at one terminal to saidfirst write driver and coupled at another terminal to said second writedriver, wherein said MTJ structure receives said first voltage to changefrom a first state to a second state and receives said second voltage tochange from said second state to said first state.
 2. The magnetic bitcell write circuit of claim 1 wherein said first voltage comprises acore network voltage, and said second voltage comprises an input/output(I/O) network voltage.
 3. The magnetic bit cell write circuit of claim 1further comprising: a plurality of columns, said at least one MTJstructure residing in one of said plurality of columns; at least oneadditional MTJ structure in each additional one of said plurality ofcolumns; at least one additional low voltage write driver coupled toeach additional one of said plurality of columns, said at least oneadditional low voltage write driver applying said first voltage; and atleast one additional high voltage write driver coupled to eachadditional one of said plurality of columns, said at least oneadditional high voltage write driver applying said second voltage,wherein said at least one additional MTJ structure receives said firstvoltage to change from said first state to said second state andreceives said second voltage to change from said second state to saidfirst state.
 4. The magnetic bit cell write circuit of claim 3 furthercomprising: first selection circuitry within said first write driver andsaid at least one additional low voltage write driver, said firstselection circuitry configured to select one of: said at least one MTJstructure and said at least one additional MTJ structure based onaddress information received during a write command; and secondselection circuitry within said second write driver and said at leastone additional low voltage write driver, said second selection circuitryconfigured to select one of: said at least one MTJ structure and said atleast one additional MTJ structure based on said address information. 5.The magnetic bit cell write circuit of claim 1 integrated into at leastone of a mobile phone, a set top box, a music player, a video player, anentertainment unit, a navigation device, a computer, a hand-heldpersonal communication systems (PCS) unit, a portable data unit, and afixed location data unit.
 6. The magnetic bit cell write circuit ofclaim 1 integrated into a semiconductor die.
 7. A magnetic random accessmemory (MRAM) comprising: a plurality of memory columns, each of saidplurality of memory columns having at least one magnetic bit cell; aplurality of source lines, each associated with a corresponding columnof said plurality of memory columns, each of said plurality of sourcelines coupled to one terminal of said at least one magnetic bit cell ofsaid corresponding column; a plurality of bit lines, each associatedwith said corresponding column, each of said plurality of bit linescoupled to another terminal of said at least one magnetic bit cell ofsaid corresponding column; a plurality of first drivers, each coupled toa corresponding source line of said plurality of source lines, saidplurality of first drivers applying a first driver voltage to changesaid at least one magnetic bit cell from a first state to a secondstate; and a plurality of second drivers, each coupled to acorresponding bit line of said plurality of bit lines, said plurality ofsecond drivers applying a second driver voltage to change said at leastone magnetic bit cell from said second state to said first state.
 8. TheMRAM of claim 7 wherein said at least one magnetic bit cell comprises: amagnetic tunnel junction (MTJ) structure; and a transistor coupled tosaid MTJ structure.
 9. The MRAM of claim 7 further comprising: firstselection circuitry within each of said plurality of first drivers, saidfirst selection circuitry controlling application of said first drivervoltage to a selected corresponding one of said plurality of sourcelines by said associated one of said plurality of first drivers based onan address signal received from a word line of said MRAM; and secondselection circuitry within each of said plurality of second drivers,said second selection circuitry controlling application of said seconddriver voltage to a selected one of said plurality of bit lines by saidassociated one of said plurality of second drivers based on said addresssignal.
 10. The MRAM of claim 7 wherein said second driver voltage ishigher than said first driver voltage.
 11. The MRAM of claim 8 whereinsaid first driver voltage is selected so as not to cause breakdown ofsaid MTJ structure, and said second driver voltage is selected so as toassure switching of said MTJ structure.
 12. The MRAM of claim 7integrated into a semiconductor die.
 13. The MRAM of claim 7 integratedinto at least one of a mobile phone, a set top box, a music player, avideo player, an entertainment unit, a navigation device, a computer, ahand-held personal communication systems (PCS) unit, a portable dataunit, and a fixed location data unit.
 14. A method for writing to amagnetic tunnel junction (MTJ) structure of a magnetic bit cell element,said method comprising: receiving a write signal on a word lineassociated with said MTJ structure; in response to said write signal,detecting a write data to be written to said MTJ structure; in responseto said write data being a first value, receiving a first voltage on abit line coupled to one terminal of said MTJ structure, said firstvoltage causing said MTJ structure to change from a first state to asecond state; and in response to said write data being a second value,receiving a second voltage on a source line coupled to another terminalof said MTJ structure, said second voltage higher than said firstvoltage and causing said MTJ structure to change from said second stateto said first state.
 15. The method of claim 14 in which said magneticbit cell element is integrated into a semiconductor die.
 16. The methodof claim 14, in which the magnetic bit cell element is integrated intoat least one of a mobile phone, a set top box, a music player, a videoplayer, an entertainment unit, a navigation device, a computer, ahand-held personal communication systems (PCS) unit, a portable dataunit, and a fixed location data unit.
 17. The method of claim 14 whereinsaid bit cell element comprises one of: a magnetic memory; and amagnetic spin-logic device.
 18. A method for writing to a magnetictunnel junction (MTJ) structure of a magnetic bit cell element, saidmethod comprising the steps of: receiving a write signal on a word lineassociated with said MTJ structure; in response to said write signal,detecting a write data to be written to said MTJ structure; in responseto said write data being a first value, receiving a first voltage on abit line coupled to one terminal of said MTJ structure, said firstvoltage causing said MTJ structure to change from a first state to asecond state; and in response to said write data being a second value,receiving a second voltage on a source line coupled to another terminalof said MTJ structure, said second voltage higher than said firstvoltage and causing said MTJ structure to change from said second stateto said first state.
 19. The method of claim 18 in which the magneticbit cell element is integrated into a semiconductor die.
 20. The methodof claim 18 in which the magnetic bit cell element is integrated into atleast one of a mobile phone, a set top box, a music player, a videoplayer, an entertainment unit, a navigation device, a computer, ahand-held personal communication systems (PCS) unit, a portable dataunit, and a fixed location data unit.
 21. The method of claim 18 whereinsaid magnetic bit cell element comprises one of: a magnetic memory; anda magnetic spin-logic device.
 22. A system for writing to a magnetictunnel junction (MTJ) structure of a magnetic bit cell element, saidsystem comprising: means for receiving a write signal on a word lineassociated with said MTJ structure; means, executable in response tosaid write signal, for detecting write data to be written to said MTJstructure; means, executable in response to said write data being afirst value, for receiving a first voltage on a bit line coupled to oneterminal of said MTJ structure, said first voltage causing said MTJstructure to change from a first state to a second state; and means,executable in response to said write data being a second value, forreceiving a second voltage on a source line coupled to another terminalof said MTJ structure, said second voltage higher than said firstvoltage and causing said MTJ structure to change from said second stateto said first state.
 23. The system of claim 22 integrated into asemiconductor die.
 24. The system of claim 22 integrated into at leastone of a mobile phone, a set top box, a music player, a video player, anentertainment unit, a navigation device, a computer, a hand-heldpersonal communication systems (PCS) unit, a portable data unit, and afixed location data unit.
 25. The system of claim 22 wherein saidmagnetic bit cell element comprises one of: a magnetic memory; and amagnetic spin-logic device.